1. Field of the Invention
The present invention relates to an image-shake correcting device for use in performing image-shake correction in a photographic camera, a video camera or the like and, more particularly, to an image-shake correcting device capable of preventing an abnormal motion of an image from occurring at the instant when the image-shake correcting device is turned on or off.
2. Description of the Related Art
In the field of photographic apparatuses such as still cameras or video cameras, various operations, such as exposure setting and focus adjustment, have heretofore been automated and an increased number of functions have been incorporated into one photographic apparatus. Accordingly, even beginners have become able to easily enjoy high-quality photography.
FIG. 1 is a block diagram showing one example of an image-shake correcting device.
The image-shake correcting device shown in FIG. 1 is arranged to cancel an image shake by optically displacing an optical axis. Specifically, if a vibration occurs in a photographic apparatus such as a still camera or a video camera, the vibration is detected by an angular-velocity detector 1 which includes an angular-velocity sensor, such as a vibration gyro, mounted in the photographic apparatus, and the angular-velocity detector 1 outputs a signal corresponding to the detected vibration. The DC component of the angular-velocity signal outputted from the angular-velocity detector 1 is cut off by a DC cutoff filter 2, and the obtained AC component, i.e., a vibration component, is passed through the DC cutoff filter 2. The DC cutoff filter 2 may use a high-pass filter (hereinafter referred to as "HPF") capable of cutting off an arbitrary band of a signal.
The angular-velocity signal outputted from the DC cutoff filter 2 is amplified to an appropriate sensitivity level by an amplifier 3, and the phase and the gain of the angular-velocity signal are corrected by a correction circuit 4 made up of an HPF and a high-frequency compensation filter. The output of the correction circuit 4 is integrated by an integrator 5, and an angular-displacement signal is inputted to an adder 6, in which the angular-displacement signal and a detected-position output are added together. In accordance with the sum outputted from the adder 6, a microcomputer (.mu.COM) 7 outputs to a driving circuit 8 an instruction to drive image correcting means (hereinafter referred to as "VAP" (variable angle prism)) 9, thereby driving the VAP 9.
In the meantime, a position of the VAP 9 is detected by a position detecting sensor 10 which monitors the position of the VAP 9, and the detected-position output of the position detecting sensor 10 is amplified by an amplifier 11. As described above, the amplified detected-position output is added to the output of the integrator 5 in the adder 6, and the sum outputted from the adder 6 is sent to the microcomputer 7 and the microcomputer 7 executes control of the VAP 9 in accordance with the inputted sum. In addition, an image-shake correcting switch 12 is provided for turning on or off the operation of the VAP 9. In accordance with the state of the image-shake correcting switch 12, a switch 13 is turned on or off, so that the operation of the VAP 9 is started or stopped.
An image-shake correcting operation which is performed by the VAP 9 and the driving circuit 8 will be described below. FIG. 2 is a schematic view showing the structure of the VAP 9.
As shown in FIG. 2, the VAP 9 includes two transparent parallel plates 740a and 740b which are opposed to each other, a transparent elastic material or inactive liquid 742 which has a high refractive index (n) and is charged into the gap between the transparent parallel plates 740a and 740b, and a sealing material 741, such as resin film, which surrounds and elastically seals the gap between the transparent parallel plates 740a and 740b so as to swingably hold the transparent parallel plates 740a and 740b. An image shake is corrected by swinging the transparent parallel plates 740a and 740b of the VAP 9 and displacing the optical axis thereof.
FIG. 3 is a schematic view showing the state of passage of an incident light flux through the VAP 9 shown in FIG. 2.
FIG. 3 is a schematic view showing the state of passage of an incident light flux 744 through the VAP 9 when the transparent parallel plate 740a is rotated by an angle .sigma. about a swinging or rotating shaft 701 (711). The light flux 744 which is made incident on the VAP 9 along an optical axis 743 is made eccentric (deflected) by an angle .phi. on the same principle as a prism.
FIG. 4 is a schematic view showing the construction of a VAP driving mechanism.
FIG. 4 shows one example of the arrangement of the VAP 9 and the driving circuit 8. In this arrangement, a voice coil is used in a driving system, and feedback control is performed on the basis of an angular displacement detected by a position detecting sensor.
As shown in FIG. 4, the VAP 9 is secured to a lens barrel 702 via a holding frame 707 so that the VAP 9 can be turned about the axis of the rotating shafts 701 and 711. A coil 712, a yoke 713 and a magnet 715 constitute a voice-coil type actuator, which can vary the apex angle of the VAP 9 about the rotating shaft 711 by causing a current to flow in the coil 712.
A slit 710 for detecting the displacement of the VAP 9 is arranged to displace its position while turning concentrically to the rotating shaft 711 together with the holding frame 707, i.e., the VAP 9. The slit 710, a PSD (position detecting element) 709 and a light-emitting diode 708 constitute a position detecting sensor. In the position detecting sensor, light emitted from the light-emitting diode 708 is received by the PSD 709 to detect the displacement of the slit 710, thereby detecting the angular displacement of the apex angle of the VAP 9.
The light flux the incident angle of which has been varied by the VAP 9 in the above-described manner is focused on the image pickup surface of an image pickup element 704 by a photographic lens unit 703. Incidentally, reference numeral 705 denotes another rotating axis which is at right angles to the axis of the rotating shafts 701 and 711 of the holding frame 707.
FIG. 5 is a block diagram showing a control circuit for controlling the driving of the VAP 9.
In the feedback system of the control circuit shown in FIG. 5, a control signal 720 for image-shake correction is supplied from the microcomputer 7 to a voice-coil type of actuator 724 through an amplifier 722 and an driver 723 for driving the actuator 724. The driver 723 drives the actuator 724 in accordance with the control signal 720, so that the actuator 724 controls the VAP 9 to vary the apex angle thereof by a predetermined amount.
The angular displacement of the apex angle of the VAP 9 is detected by the position detecting sensor 10 made up of the PSD 709 and the associated constituent elements, and the detected-position output of the position detecting sensor 10 is supplied to an adder 725. In the adder 725, the detected-position output is added to the control signal 720 which is supplied from the microcomputer 7 and of opposite polarity to that of the detected-position output. The control system operates so that the control signal 720 for image-shake correction which is outputted from the microcomputer 7 and the output signal of the position detecting sensor 10 can become equal to each other. Accordingly, the VAP 9 is driven so that the control signal 720 can coincide with the output of the position detecting sensor 10, whereby the VAP 9 is controlled so as to take the position specified by the microcomputer 7.
However, the above-described example involves a number of problems. A first problem is that the switch 13 for turning on or off the operation of the VAP 9 is provided at the front stage of a vibration-frequency detecting block which includes the phase and gain correcting circuit 4 and the integrator 5 for generating a correction target value, so that when the switch 13 is off and the operation of the VAP 9 is off, it is impossible to detect a vibration frequency, and hence to make reference to vibration data relative to the camera.
A second problem is that, although the VAP 9 is normally controlled when in a steady state, if the VAP 9 is turned on or off, an image may be moved to a great extent while the VAP 9 is switching from its off state to its on state. This is because, during the off state, a control signal for the VAP 9 is not outputted to the driving circuit 8, while the microcomputer 7 performs a correction computation on the detection output of the angular-velocity detector 1 supplied from the phase and gain correcting circuit 4 and the integrator 5, so that the VAP 9 is moved to a great extent in accordance with the magnitude of a vibration obtained at the instant when the VAP 9 is turned on.
Contrarily, if the VAP 9 is switched from the on state to the off state while it is moving, the VAP 9 tends to immediately return toward the center of the optical axis by its own driving force, so that an image is greatly disturbed.